Aline Auroux

Alumina-, silica-, and titania-supported vanadium oxide systems with V2O5 loadings ranging from 3 to 12 wt.%, corresponding to 0.02–0.09 V/(Al,Si,Ti) atomic ratios, were prepared by atomic layer deposition (ALD) and compared with the corresponding impregnated catalysts. The surface acidic properties of the supports and catalysts were investigated using ammonia adsorption microcalorimetry to determine the number and strength of the surface acid sites. Deposition of V2O5 on alumina and titania supports gave rise to catalysts with lower amounts of acid sites than the respective supports, while for the samples prepared on silica, an increase of the number of acid sites was observed after V2O5 deposition. As a common trend, the surface acid strength was greater for the ALD catalysts than for the impregnated ones, suggesting a stronger interaction of the VO species with the support centers, which act as electron attractor centers creating Lewis-like vanadium species. Redox cycles were performed, involving temperature programmed reduction (TPR) analyses separated by an oxidation treatment (TPO). The results evidenced the good reversibility of the redox behavior of the vanadium centers in every case, while significant differences were observed when comparing the temperatures of reduction (Tmax). Lower Tmax values were observed for the better dispersed vanadia catalysts. After reduction, the V centers had a final formal average oxidation state corresponding to +3 or less (+2.5 to +2). The reactivity of the vanadia systems was examined by measuring their performance for the oxidation of o-xylene to phthalic anhydride. Activity tests indicated the superior selectivity of the V2O5 systems based on the more acidic supports (Al2O3 and TiO2). The nature of the support governed the activity, and the more concentrated catalysts gave rise to improved selectivity to phthalic anhydride.

ABSTRACT Mesoporous materials (SBA-15 and Al-SBA-15 with various Si/Al ratios) were synthesized and investigated in relation to their capacity to be used as adsorbents for depollution of the contaminated air or wastewater. The compositional and the structural properties were determined by XRD, N2 isotherms, NMR, chemical analysis and XPS. The acidity and adsorption properties of the solids were checked by adsorption microcalorimetry using various basic or polluting molecules in gas phase.

Copper ion-exchanged ZSM5 zeolites have been prepared with different cooper loadings from under- to over-exchanged levels. The adsorptions of N2O and CO at 303 K have been studied using calorimetric method and infrared spectroscopy. The samples were additionally characterised by ammonia adsorption at 423 K. The active sites for both N2O and CO are Cu(I) ions, which were formed as a result of pre-treatment in vacuum at 673 K.Room temperature adsorption of nitrous oxide at low equilibrium pressures (up to 66.7 Pa) resulted in small amounts of chemisorbed N2O (<0.2 molecule per one Cu ion). Differential heats of N2O adsorption between 80 and 30 kJ/mol were obtained. Differential heats of CO adsorption between 140 and 40 kJ/mol were obtained. The obtained amounts of chemisorbed species in the investigated systems and the values of differential heats of both nitrous oxide and carbon monoxide demonstrate the dependence on the copper content.

ABSTRACT The surface acidic properties of sulfated vanadia–titania catalysts prepared by various methods were investigated by adsorption microcalorimetry, using ammonia as probe molecule. The acidic characteristics of the samples were shown to be strongly affected by the preparation method, calcination temperature, and sulfur content. The samples prepared by sol–gel and mechanical grinding exhibited higher acidity than co-precipitated samples. Moreover, increasing the calcination temperature of co-precipitated samples resulted in a decrease in surface area from 402 to 57 m2 g−1 and sulfur content from around 4 to 0.2 mass%, but up to a certain point generated a stronger acidity. The optimal calcination temperature appeared to be around 673 K.

CeO2 supported V2O5 catalysts were prepared by the wetness impregnation technique and their surface structures were characterized by O2 chemisorption, X-ray diffraction (XRD) and Raman spectroscopy (LRS). The surface acidity and basicity were measured by using microcalorimetry and infrared spectroscopy (FTIR) for the adsorption of NH3 and CO2. Temperature programmed reduction (TPR) was employed for the redox properties. In particular, isopropanol probe reactions with and without the presence of O2 were employed to provide the additional information about the surface acidity and redox properties of the catalysts. Variation of loading of V2O5 and calcination temperature brought about the changes of surface structures of dispersed vanadium species, and hence the surface acidic and redox properties. Structural characterizations indicated that V2O5 can be well dispersed on the surface of CeO2. The monolayer dispersion capacity was found to be about 8 V/nm2, corresponding to about 10% V2O5 by weight in a V2O5/CeO2 sample with the surface area of 80 m2/g. Vanadium species in the catalysts (673 K calcined) with loading lower than 10% were highly dispersed and exhibited strong surface acidity and redox ability, while higher loading resulted in the formation of significant amount of surface crystalline V2O5, which showed fairly strong surface acidity and significantly weakened redox ability. Calcination of a 10% V2O5/CeO2 at 873 K resulted in the formation of mainly CeVO4 on the surface, which showed low surface acidity and redox ability. The probe reaction seemed to suggest that the calcination at higher temperature might cause the decrease of surface acidity more than redox ability. Thus, the 10% V2O5/CeO2 catalyst calcined at 873 K exhibited much higher selectivity to benzaldehyde as compared to other V2O5/CeO2 catalysts studied in this work, although its activity for the conversion of toluene was relatively low.

This chapter discusses various studies in which calorimetry have been used to characterize zeolites, oxides, and metallic catalysts. It is apparent from the results in the literature that microcalorimetry is a very powerful technique even if it does not provide direct information about the molecular nature of the adsorbed species. The chapter describes the way the adsorption enthalpy profile of a probe molecule can be used to characterize the surface sites. Adsorption microcalorimetry permits an accurate determination of the strength and strength distribution of surface sites based on the heat of adsorption of suitable probe molecules and the differential heat versus coverage curve. A survey of applications of microcalorimetry for heterogeneous catalysis are described in the chapter with particular emphasis on the determination of the acid–base properties of zeolites, molecular sieves, metal oxides, and supported metal oxides. Additional applications of the study of reducible metal oxides and their redox character are discussed in the chapter.

This work is focused on the gas and liquid-phase adsorption of pollutants: propanol, 2-butanone, phenol and nicotine onto zeolites (H-BETA, H-ZSM-5, H-MCM-22, and clinoptilolite). Textural properties and origin of zeolites were taken into account as criteria of adsorbents selection. The aldehyde and the ketone were adsorbed in the gas phase using microcalorimetry linked to a volumetric line to evaluate adsorption. Adsorptions in water were carried out for phenol and nicotine and the evolved heats during adsorption were measured by a differential heat flow reaction calorimeter with stirring. Results are discussed in relation with the pore sizes and various interactions which could occur between the adsorbent and the adsorbate. KeywordsAdsorption-Depollution-Microcalorimetry-Organic pollutants-Zeolites

ABSTRACT The role of the adsorbed reactants and intermediates on the performance and deactivation behavior of MgZr mixed oxides as acetone self-condensation catalysts is studied in this work. DRIFT spectroscopy was used for identifying the adsorbed species and following their evolution during both acetone self-condensation reaction and thermo-desorption of pre-adsorbed reactants and products. The evolution of these species and the results of the characterization (nitrogen physisorption, temperature-programmed oxidation, and catalyst leaching) of catalysts samples taken in a continuous reactor at different temperatures (523, 623, and 723 K) and times on stream allow to determine that two concomitant deactivation causes are present in this reaction: the strong adsorption of dimers and trimers on the catalyst surface (especially important at the lowest temperature) and the formation of heavy condensation products physically deposited on the catalyst surface (more relevant at the highest temperature).

... Chem. B, 103 (1999) 1542. 7. Yu.N. Pushkar, A. Sinitsky, OO Parenago, AN Kharlanov and EV Lunina, Appl. Surf. Sci., 167 (2000) 69. RC Mehrotra, R. Bohra and DP Gaur, &amp;quot;Metal -Diketonates and Allied Derivatives&amp;quot; Academic Press, London 1978. . S. Kawagushi, Inorg. Chem. ...

The design of new amphoteric catalysts is of great interest for several industrial processes, especially those covering dehydration and dehydrogenation phenomena. Adsorption microcalorimetry was used to monitor the design of mixed oxides of zinc with Group 3 elements (aluminium, gallium, indium) with amphoteric character and enhanced specific surface area. Acid-base features were found to evolve non-linearly with the relative amounts of metal, and the strengths of the created acidic or basic sites were measured by adsorption microcalorimetry. A panel of bifunctional catalysts of various acid-base (amounts, strengths) and redox character was obtained. Besides, special interest was given to In-Zn mixed oxides for their enhanced basicity: this series of catalysts displays important basic features of high strength (q(diff) (SO₂ ads.) &amp;amp;amp;amp;amp;amp;amp;gt; 200 kJ mol(SO₂)⁻¹ in substantial amounts (1 - 2 μmol m(catalyst)⁻²), whose impact on efficiency or selectivity in catalytic dehydration/dehydrogenation can be valuable.

... CO 2. Acid sites are cations that exhibit either a high oxidation state or an unsatu-rated coordination. On the other hand, redox properties are also known to play an important role and to be related to M n+ → M ( n− 1)+ equilibrium constant and to lattice O 2 − ion lability. ...

Vanadium pentoxide catalysts have been studied in the partial oxidation reaction of ethane in the 723-843 K temperature range. The relationship between the acid-base properties and the catalytic behavior was investigated. The number and character of acidic sites of VâOâ catalysts were determined by studying the adsorption of a basic molecule using microcalorimetry. The reducibility level and the evolution of

ABSTRACT The acidity of niobic acid (NBO) has been successfully mitigated and tuned by addition of K+, Ba2+ and Nd3+ dopant species in amounts from 1 to 15 atom nm−2. The characterization of the intrinsic acid properties of the samples was performed by adsorption of NH3 in a volumetric–microcalorimetric coupled line and by temperature programmed desorption (TPD) of 2-phenylethylamine in a thermogravimetric apparatus. The K-dopant was more effective in decreasing the acidity of niobic acid than the Ba- and Nd-dopants. Complementary measurements of the effective acidity of the samples in water by base titrations with 2-phenylethylamine completed the study and revealed a different picture of the effect of the three dopants on the NBO acidity in water.All the results indicated that the K-dopant targeted more selectively the Brønsted acid sites, acting as an ion-exchanger, while Ba- and Nd-species predominantly acted on the Lewis acid sites of the NBO surface.